1. Field of the Invention
The present invention relates to liquid flow controllers and more particularly to an ultrasonic liquid flow meter that may be used in a liquid flow controller.
2. Discussion of Related Art
Various prior art techniques for measuring a flow rate of a fluid, such as a liquid or gas, flowing in a conduit, for example, a pipe or tube, are known, including thermal flow meters, coriolis force flow meters, and differential pressure flow meters. It is also known to incorporate these and other types of flow meters in feedback control loops so as to control the flow of a fluid in a system.
During manufacture of semiconductor devices, many different fluids must be precisely and accurately dispensed for use on a wafer surface being treated. For example, in a conventional apparatus, the wafers to be treated are typically positioned beneath a nozzle that then dispenses a predetermined amount of fluid (e.g., water, slurry, etchant, etc.) to coat or treat the wafer.
As the level of integration on semiconductor wafers increases, surface irregularities have become a serious problem. For example, metallization layers used to form interconnects between the various devices on a wafer may lead to substantial surface irregularities that may interfere with subsequent photolithographic steps. As a result, several processes have been developed to “flatten” or remove surface irregularities from a wafer without damaging the devices being formed on the wafer. One example of such a process is chemical-mechanical planarization (CMP) which is a polishing method that uses a planarization fluid, usually including abrasive particles and/or chemical etchants, to grind away surface irregularities as the wafer is rotated against a polishing pad.
The planarization fluids used in CMP may typically be provided to wafer manufacturers in a commercially prepackaged form, which may include two or more fluid components that are combined prior to planarizing a production run of wafers. Once the fluid components are mixed together, the resulting planarization fluid is distributed to wafer planarization machines and used to polish semiconductor wafers.
A significant disadvantage of conventional planarization systems is that the planarization fluid cannot be mixed close to where (and when) it will be used. Rather, the fluid components are mixed and stored in a storage tank, which usually requires a continually running motor, pump, or mixer to agitate the fluid and prevent the component fluids from separating, particularly, to prevent abrasive particles from separating out from the rest of the fluid. It would be advantageous to instead implement “point-of-use” mixing of the components of the planarization fluid and avoid the need for and expense of storing and continually agitating the fluids. In addition, the mixing and use of planarization fluid on an “as-needed” basis would be advantageous because the chemical etchants typically present in the planarization fluid are subject to chemical degradation and should be used soon after mixing.
The amounts and compositions of planarization fluids supplied to wafers is critical to proper manufacturing of semiconductor devices. Conventional flow meters and flow controllers lack the accuracy often needed to implement point-of-use mixing of planarization fluids. Thus, a need exists for an accurate, small-sized fluid flow controller that may be used for, among other things, controlling the fluids used in semiconductor manufacturing to allow point-of-use mixing and accurate dispensing of such fluids.